The present invention relates to a method of determining the location and thickness of passive shims in magnetic resonance magnets to maximize the magnetic field homogeneity in the bore of the magnet.
In magnetic resonance (MR) magnets a uniform magnetic field is used to polarize the hydrogen nuclei in the subject being examined. Magnetic field inhomogeneities will distort the position information in the scan volume and degrade the image quality. In chemical shift spectroscopy the chemically shifted frequency peaks are often separated by a fraction of one part per million, requiring high field homogeneity. To create a highly uniform magnetic field with an electromagnet, it is necessary to build the magnet to a carefully specified shape, and to strive to minimize the deviations from the specified shape due to manufacturing variations. The resulting magnet, however, typically requires field correction to achieve the desired level of homogeneity, due to deviations of the magnet from the design or due to the presence of ferromagnetic material in the vicinity of the magnet.
To improve field uniformity, correction coils are typically used. These coils are capable of creating different field shapes which can be superimposed on an inhomogeneous main magnetic field to perturb the main magnetic field in a manner which increases the overall field uniformity. Many sets of such coils are typically required. The state of the art magnetic resonance imaging magnet has between 10 and 20 independent sets of correction coils, each with its own power supply to provide the correct current flow. The correction coils may be resistive, superconducting or a combination of both. These coils add significantly to the cost and complexity of the magnet.
One way of removing the need for correction coils is to shim the magnet passively, using only pieces of iron to bring an initially inhomogeneous field to within imaging homogeneity specifications. The iron may be placed outside the magnet or in its bore; the latter method has the advantage of reducing the volume of iron required and thereby reducing any addition to the magnet size and weight. Such a system would be much cheaper and more reliable than the typical set of correction coils. The primary difficulty in implementing such a shimming method lies in predicting the locations and sizes of iron pieces required to shim the field. Electromagnetic coils are generally designed to produce certain terms of a spherical harmonic field expansion. Such a design criterion becomes difficult with passive shims for two reasons, the inability to specify a field reversal in the iron and the size and complexity of the groups of pieces required to produce primarily a single harmonic. Since magnetic coupling between the shims is also a complicating factor, shimming with large pieces, which inevitably become physically close, becomes difficult.
The more elegant and efficient solution is to use many small pieces of iron, placed at strategic locations, to correct the field. The iron may be configured to apply small corrections near the parts of the imaging volume which are close to optimized already, and large corrections where it isn't.
The difficulty with this solution is in predicting the necessary locations as well as the thickness of the shims since the number of possible shim locations and thicknesses is generally large.
A number of MR magnet manufacturers have investigated passively shimming their magnets from a standpoint of removing field shapes which are of a shape or magnitude which the correction coils alone cannot handle, thus rendering the magnet shimmable with the correction coils. Some manufacturers have offered magnet systems which are passively shimmed, i.e. no correction coils are used. U.S. Pat. No. 4,771,244, describes a method using linear least squares algorithm for passive shimming optimization. Even though this algorithm has been successfully used for a few passive shimming cases, in some "difficult to shim" cases, it cannot bring the field inhomgeneity down to acceptable levels.
It is an object of the invention to determine the optimum locations and thicknesses of passive shims which produce homogeneous magnetic fields within MR magnets.
It is an object with the present invention to provide a method of shimming MR magnets using only pieces of ferromagnetic material to provide homogeneous magnetic fields within the magnets.